Aromatic polycarbonates have excellent mechanical characteristics such as impact resistance as well as heat resistance and transparency and have been employed as engineering plastics in a broad range of fields, such as bottles for carbonated beverages, electronic bases (CD bases), transfer belts, etc.
Industrially established processes for producing aromatic polycarbonate include the so-called phosgene process comprising reacting an aromatic diol, e.g., bisphenol, and phosgene by interfacial polycondensation. However, the phosgene process has many disadvantages, such as high toxicity of phosgene, the necessity of handling quantities of sodium chloride as a by-product, incorporation of the sodium chloride into the polymer produced, and influences of methylene chloride usually used as a reaction solvent on the environment. That is, the phosgene process incurs high costs for taking countermeasures against these health and environmental problems.
The so-called melt process or non-phosgene process is also well known. It consists of an interesterification reaction between an aromatic diol compound and a diaryl carbonate compound. It is received that the non-phosgene process is free of the above-mentioned problems associated with the phosgene process and also is more economical.
However, the aromatic polycarbonate obtained by the non-phosgene process using, for example, bisphenol A and diphenyl carbonate generally has a higher content of terminal hydroxyl groups than that obtained by the phosgene process using, for example, bisphenol A, phosgene, and a terminal blocker, etc. As a result, the former aromatic polycarbonate is generally inferior to the latter in heat resistance and hue. The residue of the catalyst used in the non-phosgene process also has adverse influences on the aromatic polycarbonate produced.
For example, the heat resistance in terms of temperature causing a 5% weight loss on heating (Td5%) of an aromatic polycarbonate obtained by the non-phosgene process is generally lower than that of an aromatic polycarbonate prepared by the phosgene process, i.e., about 500.degree. C., sometimes lower by several tens of degrees C. or more, although it varies depending upon a kind and an amount of a catalyst for an interesterification reaction and a content of terminal hydroxyl groups of resulting aromatic polycarbonates.
Because molding of aromatic polycarbonates should be conducted at high temperatures, e.g., of around 320.degree. C., in order to decrease the melt viscosity thereof, low heat resistance of aromatic polycarbonates gives rise to problems, such as cleavage of the polymer main chain, coloration, and reduction in mechanical strength. In particular, a high molding temperature is needed for obtaining thin-walled articles such as containers having a wall thickness of from 0.3 to 0.6 mm or articles with complicated shapes. Therefore, in order that an aromatic polycarbonate obtained by the non-phosgene process may be put to practical use, improvement in heat resistance and prevention of coloration are much desired.
It has been proposed to prepare an aromatic polycarbonate having an improved hue by using, as a catalyst for interesterification, a quaternary ammonium salt or phosphonium salt, e.g., tetraphenylphosphonium tetraphenylboranate or triphenylbutylphosphonium tetraphenylboranate (see JP-B-47-17978, the term "JP-B" as used herein means an "examined published Japanese patent application") or a boron hydride compound represented by the formula R'.sub.4 PBH.sub.n R.sub.4-n (R, R': hydrocarbon group) (see U.S. Pat. Nos. 4,330,664 and 5,221,761).
However, while the aromatic polycarbonates obtained by using these catalysts have improved hue, they have insufficient molecular weight and poor heat resistance, or have a Td5% as low as 475.degree. to 480.degree. C. while having an improved hue and a high molecular weight.
U.S. Pat. No. 4,363,905 mentions production of an aromatic polycarbonate having a weight average molecular weight of 400 and a satisfactory hue by melt polycondensation of bisphenol A and diphenyl carbonate in the presence a combination of Bu.sub.4 PBr and sodium phenolate as a catalyst system for interesterification (see Column 6, Table III, Run No. VIII). However, such a low-molecular weight polycarbonate encounters difficulty in injection molding or extrusion molding. The aforementioned U.S. patent also mentions production of an aromatic polycarbonate having a weight average molecular weight of 8,400 and an excellent hue by melt polycondensation of bisphenol A and bis(o-nitrophenyl) carbonate using the same catalyst system (Run No. III). In Run No. III, because of the use of bis(o-nitrophenyl) carbonate as a starting diaryl carbonate, the cost of material is high, the heat stability during the reaction is poor, and a decomposition product is incorporated into the produced aromatic polycarbonate resulting in deterioration of the hue and low mechanical strength, such as low impact strength.